The present invention relates to an industrial apparatus for cheaply and continuously producing silicon carbide of .beta.-type crystal of fine and high purity using silica and carbon as starting materials. Particularly, the present invention relates to an improvement of the apparatus described in our U.S. patent application Ser. No. 18,939.
A particle of .beta.-type silicon carbide crystal has a rather roundish shape, so that particles of .beta.-type silicon carbide crystal are suited to a fine powdery abrasive for super precise grinding which affords faster grinding speed than conventional fine powdery abrasives without incurring deep scars on surfaces of works to be abraded. They have excellent filling property and oxidation resistance resulting also from their roundish particulate shape, so that they are suited to such uses as a filler and a coating material for improving an oxidation resistance of a heating element or a refractory made of silicon carbide and the like, particularly .beta.-type silicon carbide crystals of fine and high purity are suited to a starting material for sintered bodies such as a gas turbine element, a high temperature heat exchanger, a heat-resistant jig element, a jig element for treating melted materials, a part of high temperature furnace, a chemical equipment part and the like.
A conventional method of industrially producing the silicon carbide has been effected using a publicly known classical discontinuous Acheson type electric furnace. Thus, a sealing of the electric furnace has been difficult. Therefore, it has many environmental, labor and sanitary problems and has not been an efficient and economical method. In addition, the conventional method of using the Acheson type furnace has drawbacks that it can obtain .beta.-type silicon carbide merely in a very small quantity as a by-product in producing .alpha.-type silicon carbide and that the .beta.-type silicon carbide contains .alpha.-type silicon carbide and other impurities in high percentage and cannot be mass produced at high yield percentage.
There has hitherto been proposed many methods of continuously producing silicon carbide. For example, U.S. Pat. No. 2,178,773 Specification discloses a method wherein silicon carbide is continuously produced using a shaft kiln for a purpose of obtaining silicon carbide suitable for abrasives. In this method, in order to prevent a mixture of materials silica and carbon from agglomerating or solidifying in a cake shape due to melt of silica at a low temperature zone, the mixture of the materials is directly charged extremely little by little in a high temperature zone to completely react the materials in an upper part of the high temperature zone and the reacted materials are further heated to grow into large coarse crystals noticeable by a naked eye. However, this method of completing the SiC forming reaction at the high temperature zone and growing the crystals into large and coarse crystals is very difficult to practice, because the reaction products are sintered in the high temperature zone to form mutually agglomerated bodies or adhere on inner wall of the reaction vessel whereby a smooth transfer or descent thereof is disturbed. West German Pat. No. 1,186,447 discloses a method of continuously producing .beta.-type silicon carbide in an intermediate process using a shaft kiln, for a final purpose of obtaining .alpha.-type silicon carbide. This method uses silica sand coated with carbonaceous powders as a raw material and a special rection vessel having a gas vent hole at reaction zone, in order to prevent agglomeration of a mass of the material due to melt of the coated silica sand. However, the method has not taken into consideration a behaviour of SiO gas produced at the forming reaction of silicon carbide. Thus, the SiO gas is discharged in a great quantity from the gas vent hole arranged at the reaction vessel, therefore, not only quality and yield of the reaction product are deteriorated, but also heat efficiency is extremely lowered owing to discharges of heat of formation of the SiO gas and sensible heat of CO gas. Furthermore, a long period of stable operation cannot be expected, because the gas vent hole is clogged by the deposition of the SiO gas.
As explained above, an economical and industrial method of continuously producing silicon carbide consisting of fine .beta.-type crystal has not yet been known. However, we have proposed previously in the above U.S. patent application Ser. No. 18,939 an apparatus for producing fine silicon carbide, comprising a vertical type reaction vessel having an inlet for charging starting materials, a preheating zone, a heating zone, a cooling zone and a closable outlet for the product which are sequentially communicated in this order in vertical direction, the heating zone being made of a graphite cylinder and having an effective heating width of 0.10-0.35 m, and a means for heating the starting materials in the heating zone by an electrically indirect heating, and further comprising a heat insulating layer composed of fine powders of graphite and/or carbon and arranged on at least outside of the heating zone, whereby the materials charged from the charging inlet are preheated while descending in the preheating zone and subsequently indirectly heated and reacted with each other thus forming silicon carbide in the heating zone by the heating means while descending continuously or intermittently by their own weight and the formed silicon carbide is cooled while descending in the cooling zone and discharged from the outlet.
The above apparatus affords an extremely easy, efficient, stable and continuous operation for a long period of time when .beta.-type silicon carbide is produced by using a carbonaceous material having a relatively good reactivity, such as anthracite or the like. However, when a carbonaceous material having an extremely low ash content, for example, oil coke or the like is used for a purpose of obtaining .beta.-type silicon carbide containing especially little amount of impurities of a solid solution state, the carbonaceous material has poor reactivity to react with the SiO gas, so that an amount of the SiO gas to be discharged together with generated gases increases and the SiO gas deposits on inner wall surface of the preheating zone. The deposit on the inner wall surface of the preheating zone becomes a cause of an extremely undesirable phenomenon of preventing a smooth descent of the materials by their own weight, and the deposit is quite difficult to remove, so that a long period of stable and continuous operation has been difficult.